1. Field of the Invention
The present invention relates to a preferred plane patch antenna to be used as a GPS (Global Positioning System) antenna or the like.
2. Description of the Prior Art
In recent years, the GPS antenna is incorporated in a portable apparatus, whereby movement to constitute a portable type navigation system or to utilize it in order to acquire positional information in emergency communication between portable telephones has become active, and accordingly, a very small-sized plane patch antenna has been developed.
FIG. 5 is a perspective view showing a conventionally known plane patch antenna, and FIG. 6 is a cross-sectional view showing the plane patch antenna. As shown in these views, the conventional plane patch antenna is structured such that on one surface of a square dielectric substrate 10, there is formed a patch electrode 11, and over the other surface, there is formed a ground electrode 12. The patch electrode 11 is formed with a cutout 11a as a degenerated separate element, and in the location at some distance from the center, there is formed a feeding point 13. The structure is arranged such that power is supplied to this feeding point 13 from the ground electrode 12 through a coaxial cable 14.
In a plane patch antenna structured as described above, for the dielectric substrate 10, a ceramic material having a high dielectric constant xcex5r is generally used, and ceramic powder obtained by press molding is calcined at a desired temperature (about 1300xc2x0 C.), whereby the dielectric substrate 10 can be obtained. Also, the patch electrode 11 consists of a conductive layer of Ag or the like, which has been thick-film formed on one surface of the dielectric substrate 10 after calcination. Concretely, Ag paste of a desired shape is formed on one surface of the dielectric substrate 10 after calcination by means of screen printing, and this Ag paste is calcined at a desired temperature (about 800xc2x0 C.), whereby the patch electrode 11 can be formed.
In such a plane patch antenna, it has been known that its resonance frequency depends to a large degree on variations in dimension of the patch electrode 11 and variations in dielectric constant of the dielectric substrate 10, and as shown in FIG. 7, when a length L of one side of the patch electrode 11 becomes larger, the resonance frequency fr decreases, and as shown in FIG. 8, when the dielectric constant xcex5r of the dielectric substrate 10 becomes higher, the resonance frequency fr decreases. Therefore, to minimize these variations is very important in order to stabilize the resonance frequency. Since the dielectric substrate 10 after the calcination changes in dimension, caused by variations in particle diameter of ceramic powder, calcination temperature conditions and the like, it is difficult to restrict variations in the dielectric constant of the dielectric substrate 10. Also, since mask deviation, drips of printing and the like are feared during screen printing, it also becomes difficult to restrict variations in dimension of the patch electrode 11.
Thus, in the conventional technique described above, for example, the resonance frequency has been adjusted by cutting the patch electrode 11 before shipped as the product, but since the variations in dimension of the patch electrode 11 occur not only in the length of one side, but also in a cutout 11a, which is a degenerated separate element, when an attempt is made to adjust the resonance frequency, a circularly polarized wave generating frequency and its axial ratio will be changed, and as a result, this has led to a problem that the yield as the product would be reduced.
The present invention has been achieved in the light of the state of affairs of the prior art, and is aimed to provide a plane patch antenna through which a desired resonance frequency can be obtained with stability without requiring any troublesome frequency adjusting operation.
The present invention has been achieved by focusing attention to the fact that the variation in dimension of the dielectric substrate after calcination and the dielectric constant are in inverse proportion. According to the present invention, there is provided a plane patch antenna, in which on one surface of a dielectric substrate, there is formed a patch electrode while over the other surface the dielectric substrate, there is formed a ground electrode, wherein on the one surface of the dielectric substrate, there is formed a region partitioned through its outer edge and a difference in level, and on an entire surface of this region, the patch electrode is thick-film printed.
In the plane patch antenna structured as described above, an area of the patch electrode depends upon processing precision of a step formed on one surface of the dielectric substrate in advance, and an area of a region partitioned by this step varies with a size of the dielectric substrate after calcination. Here, the size of the dielectric substrate after calcination varies with conditions of calcination and coupling among dielectric particles, and since the degree of shrinkage is increased as the particle diameter is smaller and the calcination and coupling become closer, the outside shape of the dielectric substrate becomes smaller and the dielectric constant becomes higher. More specifically, when the outside shape of the dielectric substrate after calcination is small, the dielectric constant becomes higher to thereby decrease the resonance frequency. In this case, since the area within the region also becomes smaller in accordance with the outside shape of the dielectric substrate, the area of the patch electrode becomes smaller, whereby the resonance frequency increases. On the other hand, when the outside shape of the dielectric substrate after calcination is large, the dielectric constant decreases to thereby increase the resonance frequency, but since the area within the region becomes larger, the area of the patch electrode also becomes larger to thereby decrease the resonance frequency. Therefore, a fluctuation in the resonance frequency associated with variations in the dielectric constant and a fluctuation in the resonance frequency associated with variations in the area of the patch electrode offset each other, and a desired resonance frequency can be obtained with stability irrespective of the variations in dimension of the dielectric substrate after calcination.
In the above-described structure, if the difference in level is a concave groove continuously formed inside an outer edge of the dielectric substrate, when the patch electrode is thick-film printed within the region, it is possible to position a printing mask with the concave groove as a guide, thus improving the workability during printing.